Process of etching glass block mold faces



Patented Dec. 9, 1952 PROCESS OF ETCHING GLASS BLOCK MOLD FACES Ralph A. Johnson, Port Allegany, Pa., assignor to Pittsburgh Corning Corporation,

Allegheny County, Pa., a corporation of Pennsylvania No Drawing. Application September 15, 1950,

Serial No. 185,147

This invention relates to the etching of patterns upon face plates for molds used in the manufacture of plastic materials, and more specifically to a procedure for etching the borders of bronze face plates used in the manufacture of glass building blocks. The usual procedure in the preparation of molds for functional type glass blocks is to machine the light directional pattern faces of the molds and to etch the borders of the pattern faces where light diffusion is desired.

The art of etching metals by means of suitable solutions is old. Various metals and solutions have been used, all having at least one point of similarity in that the time of immersion in the solution was relatively short and, the depth of etch relatively shallow. When the art of etching is applied to metal molds used in the manufacture of pressed or otherwise formed plastic materials, the depth of etch is materially greater and there is a corresponding greatly increased time of immersion of the metal in the etching solution. The increased depth of etch, particularly where rapid and uniform etching is required, introduces a number of new and difiicult problems not encountered where the depth of etch was shallow and the period of immersion in the solution materially shorter. Inasmuch as the etching is done within lines of a pattern provided on the metal being etched by means of suitable masking coats, another serious problem involved is the prevention of the lifting of the masking material and side cutting, thereby destroying the lines of the desired pattern.

One object of the invention is to provide anew and novel etching procedure in which the lifting of the masking material is not influenced by the depth of etch and the length of time of immersion .of the metal in the etching solution.

Another object of the invention is the provision of a new and novel etching procedure in which the chemical reaction between the metal and the etching solution does not slow down the rate of etching, and uniformity of etch is obtained irrespective of the length of time the metal is immersed in the solution.

Another object of the invention is to provide a new and novel etching olution.

It is old in the art of the manufacture of glass building blocks where the pattern face plates of the molds were made of cast iron that those pors tions of the plate having light directional lines machined therein were covered with a suitable masking material, such as asphaltum, and the borders of the plate stippled with a similar coat to produce the desired border pattern for the de- 3 Claims. (C1. 41-43) sired degree of light diffusion. By this process the exposed unstippled surfaces of the border were usually etched with a solution comprising one part by volume of concentrated nitric acid and five parts by volume of tap water. This procedure worked very well without objectionable lifting of either of the masking coats, and a relatively rapid uniform etch obtained.

The mechanical advantages of using bronze in the mold pattern face over cast iron has been long recognized, but the disadvantages arising from attempts to provide suitable etching of the borders of these bronze face members has heretofore discouraged the use of bronze. Upon substitution of bronze for cast iron in the above described process, it was found necessary to increase the strength of the nitric acid etching solution to proportions of one part by volume of nitric acid to only three parts by volume of tap water to obtain the desired depth of etch within a reasonable time period. Thisstronger nitric acid solution lifted the stippled coating from the metal and destroyed the stippled pattern. Various other coating materials, not decomposed by nitric acid, were substituted for the asphaltum coating, among which were various commercial polyvinyl chloroacetate and chlorinated rubber coatings. These latter coatings were found to also lift during a 3-hour immersion in thestronger nitric acid solution.

The action of the asphaltum coatings under attack by nitric acid is understandable since the acid is a very powerful oxidizing agent towards most organic compounds. However, it is not so obvious why coating materials that are not decomposed by nitric acid, such as polyvinyl chloroacetate and chlorinated rubber, should also lift. One possible explanation may be that neither of these materials are completely impervious to nitric acid. A microscopic amount may pass through the film in the course of 2 or 3 hours and react with the metal to produce nitric oxide gas which would help lift the film. Also, the presence of nitric acid under the film tends to cause a concentrated solution of cupric ion creating osmotic pressure from the dilute solution on the opposite side of the film. The result is more solution forced under the film causing it to lift. The sidewise bite of nitric acid results in nitric oxide bubbles which also lift the coating.

Etching solutions comprising ferric chloride and nitric acid, being also well known in the art, were then tried with the above masking materials and in different concentrations. In all cases the various commercial masking materials used lifted to a greater or lesser degree, indicating that such an etching solution was undesirable. No doubt, special masking compounds could be prepared but this would increase the cost of the process. Furthermore, nitric acid, when present in any appreciable amount, always exhibited a pronounced tendency for side cutting which contributed directly to the lifting of the masking coating; it being understood that although a slight lifting or. under cutting in the border might not be serious, any undesired etching of the central pattern faces of the mold was most undesirable.

In all of the foregoing tests therequired depth of etch was .008 to .010 inch, and the time of immersion of the metal in the solution was 2- to 3 hours.

Solutions of ferric chloride in water for etching metals have been tried. Such ferric chloride solutions in general were characterized by their varying degree of satisfaction in regard to the speed of etching in relation to the time of immersion. In the case of a long immersion time, i. eiupwards to 3 hours, the use of'such solutions was most unsatisfactory by reason of the pronounced tendency of the reaction to slow down and finally stop after a reasonably short interval.

This tendency of the reaction when using ferric chloride etching solutions to slow down after short periods resulted in various efforts to doctor the solutions to overcome this tendency, some of which have been relatively successful when the depth of etch was not too deep and a relatively short period of immersion of the metal in the etching solution would obtain the required depth of etch. Where, however, the depthof etch was such as is involved in mold face plates for the present purposes, and in all cases where the time of immersion in the solution was relatively long, the modified ferric chloride solutions have not proved satisfactory.

Ferric chloride solutions of various strengths were tested. All of the various strengths of solution, however, involved the difiiculty of the reaction slowing down whenever the surface being etched was placed facing upwardly in the solution. Agitation of the solution, either by means of a suitable vibrator or a stirring mechanism, had little effect in preventing the inherentcharacteristic of the solution reaction to slow down.

The most suitable Way found to remove the deposit forming on the face of the surface being etched was to remove the plate and either wipe the plate clean or wash it with water. This, however,- was economically undesirable and the plate was thenplaced in the .solution with the surface to be etched facing downwardly in the solution and-suspended above the bottom of the tank containing. the solution. In this position the desired depth of etch, using a 40? to.50 Baum'solution, was accomplished in about, 3 hours. It was observed, however, that when-the plate was held in a horizontal position and immersed into the etching solution, considerable air was trapped beneath the surface of the plate and formed bubbles on the underside of the plate which inhibited contact of the solution with the metal, resulting in lack of uniformity in depth of etch. Here again it was observed that agitation of the solution or vibration of the plate did not give uniformly improved results, probably because of the air bubbles.

A greater depth of tank was then provided and the faceplate was held in substantially vertical positionwhile being immersed wholly below the level of a 42 Baum ferric chloride etching solution. The plate was then turned to bring the surface to be etched face downward and suspended above the bottom of the tank, as previously described. Following this procedure, the desired depth and uniformity of etch without agitation or vibration was obtained in about 3 hours and without the air bubbles. The use of a vibrator in contact with theimmersed plate increased the speed of etching about 75% in the same elapsed time. In all cases, withor without use of the vibrator or other agitatin means, the plate with the surface being etched faced downwardly in the solution gave uniform results.

I claim:

1. The method of deep etching border patterns upon machined pattern faces of bronze glass block molds, which consists of coating the machined faces with a continuous asphaltum masking coat, stippling the desired pattern of a masking coat upon the borders, immersing the mold with the border pattern face down in a con centrated ferric chloride solution, continuously suspending the mold above the bottom of the solution container for 2 to 3 hours, removing the mold and washing in clear water before removing the masking coats.

2. The method of etching border patterns upon the patterned face plates of bronze glass molds which consists in masking the metal face of the plate to provide the desired border pattern in exposed metal, immersing the entire plate while in a vertical position into a concentrated solution of ferric chloride, rotating the plate beneath the surface of the solution to turn the masked face downward, then suspending the plate above the bottom of the container for the solution until the desired depth of etch is obtained.

3. The method as in claim 2 in which the suspended plate is subjected to impulses from a vibrating device to rapidly dislodge all ferrous hydroxide precipitate from the vicinity of the face being etched.

RALPH A. JOHNSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,313,233 Grass Aug. 12, 1919 2,026,603 Zarse Jan. 7. 1936 

1. THE METHOD OF DEEP ETCHING BORDER PATTERNS UPON MACHINED PATTERN FACES OF BRONZE GLASS BLOCK MOLDS, WHICH CONSISTS OF COATING THE MACHINED FACES WITH A CONTINUOUS ASPHALTUM MASKING COAT, STIPPING THE DESIRED PATTERN OF A MASKING COAT, UPON THE BORDERS IMMERSING THE MOLD WITH THE BORDER PATTERN FACE DOWN IN A CONCENTRATED FERRIC CHLORIDE SOLUTION, CONTINUOUSLY SUSPENDING THE MOLD ABOVE THE BOTTOM OF THE SOLUTION CONTAINER FOR 2 TO 3 HOURS, REMOVING THE MOLD AND WASHING IN CLEAR WATER BEFORE REMOVING THE MASKING COATS. 